1. Field of the Invention
The present invention generally relates to magnetic recording media and magnetic storage apparatuses, and more particularly to a magnetic recording medium having magnetic layers which are antiferromagnetically coupled via a nonmagnetic spacer layer, and to a magnetic storage apparatus which uses such a magnetic recording medium.
2. Description of the Related Art
The storage capacity of longitudinal magnetic recording media has been rising rapidly due to reduction of media noise, the development of high-sensitivity spin-valve heads, and high-magnetization write heads. Recording densities above 100 Gbits/inch2 have been demonstrated and such high recording densities are on the verge of being applied for a commercial hard disk drive. The demand for greater recording densities for better performing computers is, however, showing an increasing trend imposing greater challenges for the recording media and other component design.
Lowering media noise involves writing a sharper magnetic transition in the magnetic layer. This is generally achieved by increasing the media coercivity, decreasing the thickness of magnetic layer, decreasing the grain size and grain size distribution of the magnetic layer, and magnetically isolating the grains of the magnetic layer. Decreasing the grain size or decreasing the media thickness, however, adversely affect the thermal stability of the recording media. The thermal stability of the magnetic layer is normally represented by how large the factor KuV/kT is, where Ku denotes the magnetic anisotropy, V denotes the volume of the grain, T denotes the temperature, and k denotes the physical constant known as the Boltzmann constant.
In order to have small grains which are thermally stable, the magnetic anisotropy Ku has to be increased. The magnetic anisotropy field Hk is defined as Hk=2 Ku/Ms, where Ms denotes the saturation magnetization. A large magnetic anisotropy field Hk means a large coercivity Hc at the nanosecond regime where normally the writing of the information occurs for a high-density media with high-data transfer rates. High coercivity Hc at writing frequencies puts severe limitations on the write heads, as a large write current is required in order to write information on such media. Write currents, which can be produced by write heads, are severely limited due to difficulties in developing high magnetic moment write heads. The overwrite performance, which is the ability to write new data on previously written data, is worse for recording media with higher magnetic anisotropy field Hk. Recording media with a higher magnetic anisotropy Ku increases the magnetic anisotropy field Hk and thus restricts the overwrite performance.
As described above, there is need to decrease the grain sizes of the magnetic layer and the thickness of the magnetic layer in order to achieve low media noise and hence a high density recording performance. However, such reduction of grain size and magnetic layer thickness deteriorates the thermal stability of the recording medium. In order to improve the thermal stability without effecting the overwrite performance, a synthetic ferrimagnetic media (SFM) has been proposed and demonstrated, as discussed by Abarra et al., in Applied Physics Letters, Vol.77, Page 2581, October 2000.
A synthetic ferrimagnetic media (SFM) has at least one pair of magnetic layers, which are separated by a nonmagnetic spacer made of Ru or the like. The magnetization of the upper magnetic layer is partially cancelled by the lower magnetic layer which functions as an initial stabilizing layer. While the read head is sensitive only to the effective magnetization, a total volume of the two magnetic layers contributes to the thermal stability. Using this concept of the SFM, the signal-to-noise ratio (SNR) and thermal stability of the recording media are greatly improved. However, it is desirable to further improve the thermal stability so as to achieve higher recording densities.